1. Field of the Invention
The present invention relates to an ink jet ink excellent in print durability and the like, an ink jet recording method, an ink cartridge, a recording unit, and an ink jet recording apparatus.
2. Related Background Art
An ink jet recording method is a recording method involving applying a small ink droplet to any one of recording media such as plain paper and glossy media to form an image, and has become rapidly widespread owing to a reduction in its cost and an improvement in its recording rate. With the rapid spread of a digital camera in addition to an improvement in quality of an image recorded by this method, the method has been generally used as a method of outputting a photographic image comparable to silver halide photograph.
In recent years, image quality has undergone an improvement never possible before owing to, for example, an extreme reduction in size of an ink droplet and an improvement of color ranges involved in the introduction of multi-color inks. Meanwhile, there have been growing demands for a coloring material and ink, so more stringent properties have been required in terms of an improvement of coloring property and reliability such as sticking property or ejection stability.
As compared to silver halide photograph, the ink jet recording method is problematic in terms of, for example, the image storage stability of the resultant recorded article. In general, the recorded article obtained by means of the ink jet recording method is inferior in image storage stability to silver halide photograph, and involves the emergence of a problem in that a coloring material on the recorded article is apt to deteriorate to cause a change in color tone of an image and the color fading of the image when the recorded article is exposed to light, humidity, heat, an environmental gas present in the air, or the like for a long period of time. A large number of proposals have been conventionally made with a view to solving the above problem.
For example, there has been proposed the use of a coloring material having a specific anthrapyridone structure to improve fastness properties (see, for example, JP 2002-332419 A and JP 2003-192930 A). The fastness properties of a recorded article obtained by means of an ink containing the above dye are very excellent.
However, when the ink containing the above coloring material is used as an ink applicable to an ink jet recording apparatus, it has been difficult to secure reliability in some cases. In particular, an ink jet recording method involving applying thermal energy to ink to eject the ink may involve the occurrence of a problem in that kogation occurs at the heating portion of a recording head (the heating portion itself when the heating portion is in direct contact with a liquid, or a protective layer when the heating portion has at least one protective layer on its surface) or a problem in that long term continuous printing results in the deterioration (dissolution) of a heating portion surface in contact with liquid; or the disconnection of wiring for applying a voltage to the heating portion.
The inventors of the present invention have studied a cause for such phenomenon as described above occurring in an ink containing the above coloring material excellent in fastness properties. As a result, they have found that the phenomenon depends on a specific site in a coloring material structure and a specific ion in the ink. The specific site is for example a substituent that easily releases a hydrogen ion such as a hydroxyl group bonded to a triazine ring, and the specific ion is for example a hydroxide ion.
When a coloring material having, in its structure, a substituent that easily releases a hydrogen ion is turned into an aqueous solution (ink), the ink may have a buffer region (region maintaining a approximately constant hydrogen ion concentration). The ink having a buffer region has advantages such as an ability to suppress the decomposition of a coloring material because the ink maintains its pH at a constant level even when the ink is left in various environments at the time of, for example, materials flow.
However, as described above, when long term continuous printing is performed by means of an ink having a buffer region in an ink jet recording apparatus having a heating portion containing at least a metal and/or a metal oxide, the metal and/or the metal oxide in the surface at which the heating portion in contact with a liquid and a hydroxide ion react with each other every time the ink is ejected (thermal energy is applied to the ink). As a result, problems such as the deterioration (dissolution) of the heating portion surface in contact with liquid and the disconnection of wiring for applying a voltage to the heating portion occur.
One possible approach to solving the above problems is to reduce the coloring material concentration in the ink, for example, to reduce the coloring material content to be lower than 3%. However, a reduction in dye concentration in ink may make it impossible to provide a recorded article having a high density.
The inventors of the present invention have solved the above problems by contriving the composition of ink (such as the incorporation of a specific compound into the ink) to find a method with which the coloring material having a substituent that easily releases a hydrogen ion can be successfully used. The present invention has been made on the basis of this finding.